溶液特性及共存物對(duì)納濾膜去除水中全氟辛酸的影響

王佳璇,胡御寧,岳向雷,王 磊,劉 喆

溶液特性及共存物對(duì)納濾膜去除水中全氟辛酸的影響

王佳璇1,2,3,4*,胡御寧1,岳向雷4,5,王 磊4,5,劉 喆5

(1.西安科技大學(xué)建筑與土木工程學(xué)院,陜西 西安 710054；2.西安科技大學(xué)礦業(yè)工程博士后流動(dòng)站,陜西 西安 710054；3.榆林市生態(tài)環(huán)境局,陜西 榆林 719000；4.西安建筑科技大學(xué)陜西省膜分離技術(shù)研究院,陜西 西安 710055；5.西安建筑科技大學(xué)環(huán)境與市政工程學(xué)院,陜西 西安 710055)

利用聚酰胺復(fù)合納濾(NF)膜去除水中的持久性有機(jī)污染物－全氟辛酸(PFOA),分別研究了PFOA濃度、溶液離子強(qiáng)度、pH值、典型大分子有機(jī)物－牛血清蛋白(BSA)以及BSA與Na+共存對(duì)PFOA截留率的影響.結(jié)果表明,在0.8MPa下過濾24h,隨著PFOA濃度增大,PFOA截留率提高;溶液離子強(qiáng)度、pH值越高,PFOA截留率越高;當(dāng)PFOA原液中加入BSA時(shí),BSA不但會(huì)堵塞一部分膜孔,同時(shí)會(huì)在膜面上形成有機(jī)污染層,增大膜面負(fù)電量,這樣一方面增強(qiáng)NF膜的篩分作用,另一方面增強(qiáng)其靜電排斥作用;當(dāng)BSA與Na+共存時(shí),NF膜的孔堵程度更加嚴(yán)重,且Na+使形成的有機(jī)污染層更加密實(shí),NF膜的篩分能力進(jìn)一步增強(qiáng).

全氟辛酸；聚酰胺復(fù)合納濾膜；溶液特性；牛血清蛋白；無機(jī)離子

全氟辛酸(PFOA)是一種常見的全氟化合物(PFCs).因其C-F鍵極性較強(qiáng)且鍵能較高(460kJ/ mol)[1],具有較強(qiáng)的熱穩(wěn)定性、化學(xué)穩(wěn)定性、疏水疏油等特性[2-3],大量應(yīng)用在工業(yè)生產(chǎn)和消費(fèi)產(chǎn)品中[4-5](祝凌燕 and 林加華 2008).目前,PFOA廣泛存在于地表水[6]、地下水[7]、垃圾滲濾液[8]和污水處理廠出水中[9].在動(dòng)物和人體中檢測(cè)到PFOA會(huì)對(duì)生物體的免疫系統(tǒng)、肝臟以及腎臟等造成毒害作用[10],特別是危害新生嬰兒的成長[11-12].聯(lián)合國化學(xué)監(jiān)管機(jī)構(gòu)于2019年發(fā)布了PFOA的使用禁令[13].當(dāng)前去除PFOA的主要方法有電化學(xué)氧化[14],光化學(xué)降解[15],超聲波降解[16]等化學(xué)方法以及吸附[17]、膜分離[18]等物理方法.

納濾(NF)是一種壓力驅(qū)動(dòng)的膜分離技術(shù),具有成本低、能耗低、環(huán)境友好以及截留率高等優(yōu)勢(shì)[19],且表面通常荷電,可以截留多價(jià)離子和分子量介于200～1000Da的小分子有機(jī)物.因此,NF技術(shù)在去除水中PFCs方面具有極大的應(yīng)用潛力.Zhao等[20]考察了鈉離子、鈣離子和鐵離子對(duì)ESNA1-K1納濾膜去除全氟辛烷磺酸(PFOS)的影響,結(jié)果表明NF膜可有效去除水中PFOS,且去除率與離子價(jià)態(tài)有關(guān). Wang等[21]研究發(fā)現(xiàn)聚哌嗪酰胺復(fù)合NF膜對(duì)PFOS的截留率較高,且受溶液離子強(qiáng)度、大分子有機(jī)物的影響.PFOA和PFOS都屬于PFCs,因此,不同溶液組分、含量和物化性質(zhì)都可能影響NF膜去除PFOA.作為微生物胞外聚合物(EPS)的主要組分之一,蛋白質(zhì)類物質(zhì)普遍存在于各類天然水中[22],目前蛋白質(zhì)及蛋白質(zhì)與常見無機(jī)離子共存對(duì)NF膜去除PFOA的影響研究較少.

本研究利用聚酰胺復(fù)合NF膜去除水中的PFOA,選用牛血清蛋白(BSA)模擬天然水體中廣泛存在的蛋白質(zhì)類物質(zhì),考察了PFOA濃度、pH值、溶液離子強(qiáng)度、BSA以及BSA與Na+共存對(duì)NF膜去除PFOA的影響.

1 材料與方法

1.1 實(shí)驗(yàn)試劑和材料

PFOA(純度397%)和BSA(純度397%)購自美國Sigma-aldrich公司.氯化鈉、氫氧化鈉、鹽酸購自天津天力化學(xué)試劑有限公司.聚乙二醇(PEG600)、曲拉通均購自天津市科密歐化學(xué)試劑廠.N,N -二甲基乙酰胺(DMAc)、丙烯酸(AA)均購自天津市福晨化學(xué)試劑廠.聚乙烯吡咯烷酮(PVP k30)購自上海藍(lán)季科技發(fā)展有限公司.間苯二胺(MPD)、均苯三甲酰氯(TMC,純度98%)均購自上海阿拉丁生化科技股份有限公司.除特別說明外,以上試劑均為分析純.乙腈(色譜純)購自美國Thermo-Fisher公司.聚醚砜(PSF,E6010)購自德國BSAF公司.

利用去離子水和30mg/L PFOA、1g/L BSA儲(chǔ)備液以及無機(jī)鹽配制PFOA污染原液.并利用1mol/L的氫氧化鈉和鹽酸溶液調(diào)節(jié)各原液的pH值.除溶液pH值對(duì)PFOA去除影響的實(shí)驗(yàn)以外,其他實(shí)驗(yàn)原液pH值均調(diào)為(7.5±0.1).除PFOA濃度對(duì)NF膜去除PFOA影響的實(shí)驗(yàn)之外,其他所有PFOA實(shí)驗(yàn)原液中PFOA的初始濃度均為10 μg/L.

1.2 NF膜的制備

首先采用凝膠相轉(zhuǎn)化法制備NF基膜.按照一定比例將DMAc(72wt%)、曲拉通(2wt%)、PEG600 (2wt%)、丙烯酸(2wt%)、PVP k30(4wt%)和PSF (18wt%)混合后制備均質(zhì)的鑄膜液.然后將靜置脫泡后的鑄膜液傾倒在無紡布上,使用刮膜器涂覆均勻.隨后立即浸入到(22±1)℃恒溫水浴中進(jìn)行分相,即得到聚砜基膜.隨后通過界面聚合法制備NF膜功能層,先將制備好的基膜晾干,然后浸入MPD的水相溶液中,一段時(shí)間后取出晾干,再將膜浸入TMC的正己烷溶液中,一段時(shí)間后取出,最后將膜在80℃下進(jìn)行熱處理,即得到聚酰胺復(fù)合NF膜.

聚酰胺復(fù)合NF膜純水通量為(30±2)L/(m2?h), 2g/L MgSO4截留率為90%±2%.

1.3 實(shí)驗(yàn)裝置和步驟

本研究采用小型平板NF錯(cuò)流過濾系統(tǒng)進(jìn)行PFOA分離實(shí)驗(yàn).系統(tǒng)有效膜面積為65cm2,錯(cuò)流速度為4.5cm/s,系統(tǒng)壓力為0.8MPa.

具體操作過程如下:首先,將干凈的新NF膜安裝在過濾系統(tǒng)內(nèi),用去離子水預(yù)壓24h直到膜通量穩(wěn)定.若PFOA實(shí)驗(yàn)原液中含有無機(jī)鹽,則利用相應(yīng)的無機(jī)鹽溶液平衡系統(tǒng)3h,直到膜通量穩(wěn)定;若PFOA實(shí)驗(yàn)原液中不含無機(jī)鹽,則不進(jìn)行此步.記錄穩(wěn)定后的膜通量0為初始通量.之后換上新制備的PFOA實(shí)驗(yàn)原液過濾24h,每隔一段時(shí)間對(duì)原液和濾液進(jìn)行取樣,并記錄實(shí)時(shí)膜通量J.測(cè)取各個(gè)時(shí)刻水樣中PFOA的濃度,計(jì)算截留率.過濾實(shí)驗(yàn)采用全回流模式,為減小實(shí)驗(yàn)誤差,每個(gè)實(shí)驗(yàn)均進(jìn)行3次.膜通量按照式(1)計(jì)算,PFOA截留率按照式(2)計(jì)算.將J/0的比值作為NF膜的比通量.

式中:為膜通量,L/(m2·h);為濾液體積,L;為有效膜面積,m2;為過濾時(shí)間,h.

式中:P為濾液中PFOA濃度,μg/L;f為原液中PFOA濃度,μg/L.

1.4 分析方法

使用超高效液相色譜—三重四級(jí)桿質(zhì)譜聯(lián)用儀(UPLC-MS/MS,沃特世,美國)測(cè)定樣品中PFOA的濃度[23].色譜柱為BEH C18(ACQUITY UPLC,柱長:100mm,內(nèi)徑:2.1mm,填料直徑:1.7μm,沃特世,美國),流動(dòng)相為乙腈/超純水,梯度洗脫,流速為0.2mL/ min.使用固體表面Zeta電位儀(SurPASS,安東帕,奧地利)測(cè)量NF膜的表面電位.采用Zeta電位儀(ZS90Zeta,馬爾文,英國)測(cè)定不同污染液的Zeta電位.使用掃描電子顯微鏡(SEM,JSM-6510LV,捷歐路,日本)觀察受污染的NF膜表面形貌.

2 結(jié)果與討論

2.1 PFOA濃度對(duì)聚酰胺復(fù)合NF膜去除PFOA的影響

從圖1(a)可以發(fā)現(xiàn),在不同PFOA濃度下,實(shí)驗(yàn)初期PFOA截留率高但下降速率快,隨后PFOA截留率的變化速率變緩.當(dāng)過濾至24h時(shí),PFOA截留率趨于穩(wěn)定,在5,10,25,50μg/L的濃度下,PFOA的截留率依次為77.4%,81.2%,83.8%,85.9%.

產(chǎn)生以上結(jié)果主要?dú)w于以下幾個(gè)原因:首先,實(shí)驗(yàn)使用的NF膜為新膜,實(shí)驗(yàn)初期,膜上大量的活性位點(diǎn)能夠吸附PFOA,隨時(shí)間的推移,NF膜吸附容量不斷下降,此過程中PFOA的截留率快速降低,直至達(dá)到吸附平衡后吸附量基本保持不變,此時(shí)吸附作用便不再影響PFOA的截留率.這與金葉等[24]利用NF270膜去除鄰苯二甲酸酯時(shí),得出吸附作用只在過濾初期發(fā)揮作用的結(jié)論一致.其次,由于PFOA的分子量(414Da)[25]大于NF膜的截留分子量(約350Da),根據(jù)篩分機(jī)理,NF膜可以有效截留PFOA.再者,PFOA的pa為2.8[26],在溶液pH=7.5的條件下,PFOA解離呈負(fù)電,且聚酰胺復(fù)合NF膜表面帶負(fù)電(-11.70mV,表1),因此PFOA與NF膜之間存在靜電排斥力,有助于PFOA截留.

從圖1可以看出,當(dāng)過濾至24h時(shí),隨著PFOA的濃度從5μg/L增加到50μg/L,NF膜對(duì)PFOA的截留率也明顯增加,而NF膜通量卻逐漸減小.這是因?yàn)樵谶^濾過程中,PFOA會(huì)堵塞膜孔,而且PFOA的濃度越高,膜孔窄化和堵塞越嚴(yán)重,NF膜的篩分能力就越強(qiáng).此外,膜面濃差極化作用隨著PFOA濃度的升高而增強(qiáng)[27].結(jié)合不同PFOA濃度下,NF膜對(duì)PFOA的去除特性可知,濃差極化作用并未導(dǎo)致PFOA截留率降低.因此,NF膜去除PFOA主要是吸附作用、篩分作用和靜電排斥作用共同作用的結(jié)果.

2.2 溶液pH值對(duì)聚酰胺復(fù)合NF膜去除PFOA的影響

圖2 pH值對(duì)NF膜截留率的影響

由圖2可知,在實(shí)驗(yàn)進(jìn)行的各個(gè)時(shí)刻,PFOA的截留率與溶液的pH值呈正相關(guān).這是因?yàn)槿芤簆H值會(huì)影響聚酰胺復(fù)合NF膜表面的帶電情況(表1),從而很大程度地影響聚酰胺復(fù)合NF膜的性能.上文提到,當(dāng)溶液的pH值大于2.8時(shí),PFOA發(fā)生解離且主要以荷負(fù)電的形式存在.而聚酰胺復(fù)合NF膜表面含有大量的氨基[28]和羧基[29],在pH值較低時(shí),膜表面的氨基發(fā)生質(zhì)子化過程[28],形成荷正電的-NH3+,導(dǎo)致膜表面負(fù)電量減小,減弱了膜面與PFOA之間的靜電排斥力;相反,當(dāng)pH值較高時(shí),膜面羧基發(fā)生去質(zhì)子化過程[29],且pH值越大,去質(zhì)子化的羧基數(shù)量越多,則膜面負(fù)電量越大,強(qiáng)化了NF膜與PFOA之間的靜電排斥力.因此隨著pH值的不斷升高,質(zhì)子化的氨基數(shù)量不斷減少,去質(zhì)子化的羧基數(shù)量不斷增加,且從表1可以看出膜面所帶負(fù)電量隨pH值的升高而增大,從而提高了NF膜對(duì)PFOA的靜電排斥作用,PFOA截留率升高.

表1 不同pH值條件下聚酰胺復(fù)合NF膜表面Zeta電位

2.3 溶液離子強(qiáng)度對(duì)聚酰胺復(fù)合NF膜去除PFOA的影響

由圖3(a)可知,當(dāng)PFOA原液中存在無機(jī)鹽時(shí),PFOA的截留率有顯著提高,在實(shí)驗(yàn)進(jìn)行的各個(gè)時(shí)刻,PFOA的截留率與溶液離子強(qiáng)度呈正相關(guān).當(dāng)過濾至24h,溶液離子強(qiáng)度分別為0,10,50, 100mmol/L時(shí),PFOA截留率依次為81.2%,83.4%, 86.0%,86.8%,溶液離子強(qiáng)度由0mmol/L增加到10mmol/L,PFOA截留率提高2.2%,溶液離子強(qiáng)度由10mmol/L增加到50mmol/L,PFOA截留率提高2.6%.此外,從不同溶液離子強(qiáng)度下,過濾終點(diǎn)時(shí)的NF膜通量(圖3(b))可以看出,隨著原液的離子強(qiáng)度從0mmol/L增大到100mmol/L,NF膜通量逐漸減小.這是因?yàn)槟け砻婧湍た字械目购怆x子濃度都隨著溶液離子強(qiáng)度的增加而升高,導(dǎo)致膜孔內(nèi)的靜電排斥力減小[21].因此,膜孔收縮,膜滲透通量下降,同時(shí)NF膜的篩分能力得到增強(qiáng)[30].由表2可以看出,溶液離子強(qiáng)度越大,膜面負(fù)電量越少,并且溶液離子強(qiáng)度由0mmol/L增加到50mmol/L時(shí),膜面負(fù)電量降低幅度較大,所以膜表面對(duì)PFOA的靜電排斥力減弱較快.結(jié)合PFOA的截留率分析可知,在本試驗(yàn)的溶液離子強(qiáng)度條件下,篩分作用主導(dǎo)PFOA截留.另一方面,由于Cl-的半徑[31]小于PFOA-[32],因此溶液中Cl-擴(kuò)散能力大于PFOA-,根據(jù)道南效應(yīng),為了維持原液的電性平衡[33],更多的PFOA會(huì)被NF膜截留.

表2 不同離子強(qiáng)度條件下聚酰胺復(fù)合NF膜表面Zeta電位

2.4 BSA及BSA與Na+共存對(duì)聚酰胺復(fù)合NF膜去除PFOA的影響

分別配制含有BSA、BSA和NaCl的2種PFOA原液,使得BSA濃度為10mg/L, Na+濃度為10mmol/L.

由圖4可知,過濾初期各實(shí)驗(yàn)條件下PFOA的截留率隨時(shí)間的推移均逐漸減小,當(dāng)過濾至24h時(shí)基本趨于穩(wěn)定狀態(tài),原液為PFOA,PFOA+BSA,PFOA+ BSA+Na+時(shí),過濾終點(diǎn)PFOA的截留率分別為81.2%,82.6%,84.6%.可見BSA或BSA與Na+共存都可以不同程度地提高PFOA的截留率.從圖5中可以看出,新膜(圖5(a))表面干凈平整;當(dāng)加入BSA時(shí)(圖5(b)),過濾后的NF膜表面明顯被一層污染物完全覆蓋,污染層很致密;當(dāng)加入BSA和Na+時(shí)(圖5(c)),過濾后的NF膜表面覆蓋的污染物量更多,且污染物尺寸更大,團(tuán)聚更明顯,污染層也非常密實(shí).此外,不同原液條件下NF膜的通量衰減率依次為新膜

造成以上現(xiàn)象的主要原因是:(1)當(dāng)污染液為PFOA+BSA時(shí),一部分有機(jī)物造成膜孔窄化和堵塞,減小NF膜孔徑,另一部分有機(jī)物沉積到膜表面,形成致密的有機(jī)污染層(圖5(b)),從而強(qiáng)化NF膜的篩分作用,提高PFOA的截留率[34].如表3所示,當(dāng)PFOA溶液中加入BSA時(shí),NF膜表面Zeta電位從-11.70mV升高到-14.56mV(負(fù)號(hào)表示膜面所帶電性),負(fù)電量增加,因此NF膜對(duì)PFOA的靜電排斥作用增強(qiáng),PFOA截留率增大.BSA的分子量大約為66.64kDa[35],具有復(fù)雜的空間結(jié)構(gòu)和多種官能團(tuán),對(duì)PFOA有吸附作用[36],因此被吸附的PFOA隨BSA一起被NF膜截留,從而提高PFOA的截留率.(2)當(dāng)污染液為PFOA+ BSA+Na+時(shí),溶液中的PFOA和BSA[37]均帶負(fù)電,靜電排斥作用阻礙了有機(jī)物在荷負(fù)電的NF膜表面沉積,而Na+會(huì)中和NF膜表面及有機(jī)物分子的部分負(fù)電荷,使膜－有機(jī)物、有機(jī)物－有機(jī)物之間的靜電排斥力減小,因此與原液中僅存在BSA相比(圖5(b)),BSA與Na+共存時(shí)NF膜面沉積的污染物更多(圖5(c)),尺寸更大,團(tuán)聚明顯,污染層更加密實(shí),從而更有效強(qiáng)化NF膜的篩分作用,提高PFOA的截留率[38].通過對(duì)比3種原液條件下NF膜表面Zeta電位關(guān)系(表3),結(jié)合各條件下PFOA的截留率可知,靜電排斥作用不是BSA和Na+共存時(shí)PFOA截留率上升的主要原因.Na+存在時(shí), PFOA與BSA之間的排斥力減小,更容易被BSA吸附,相當(dāng)于PFOA的尺寸變大,從而與BSA被NF膜一并截留.

表3 不同原液條件下聚酰胺復(fù)合NF膜表面Zeta電位

3 結(jié)論

3.1 聚酰胺復(fù)合NF膜可有效去除水中的PFOA,且PFOA濃度越高,NF膜對(duì)PFOA的截留率越大,這主要是NF膜的吸附作用、篩分作用和靜電排斥作用共同作用的結(jié)果.

3.2 隨著溶液pH值的增加,PFOA截留率增大.這是因?yàn)槿芤簆H值越大,NF膜表面的負(fù)電量越多,膜與PFOA分子間的靜電斥力越強(qiáng),PFOA截留率越大.

3.3 當(dāng)PFOA原液中加入無機(jī)鹽時(shí),PFOA的截留率增加,且溶液離子強(qiáng)度越高,PFOA截留率越大.這是因?yàn)槟け砻婧湍た字械目购怆x子濃度都隨著溶液離子強(qiáng)度的增加而升高,使膜孔內(nèi)靜電排斥作用減弱,從而引起膜孔收縮,加強(qiáng)NF膜的篩分作用,提高PFOA的截留率.

3.4 當(dāng)PFOA原液中存在BSA時(shí),PFOA的截留率提高了1.37%;當(dāng)溶液中同時(shí)存在BSA和Na+時(shí), PFOA的截留率又提高了2.04%.這是因?yàn)锽SA堵塞膜孔,增強(qiáng)NF膜的篩分作用,且膜面負(fù)電量增加,增強(qiáng)靜電排斥作用.此外,BSA吸附PFOA,增大PFOA的體積,提高NF膜的篩分作用,故PFOA的截留率升高.當(dāng)BSA與Na+共存時(shí),Na+中和了BSA及NF膜表面的負(fù)電荷,使形成的有機(jī)污染層更加厚且密實(shí),增強(qiáng)了NF膜的篩分作用,降低了靜電排斥作用,而PFOA去除率進(jìn)一步升高證實(shí)了此時(shí)篩分作用主導(dǎo)PFOA截留.

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Effects of solution characteristics and coexisted substances on the removal of perfluorooctanoic acid from water by nanofiltration membrane.

WANG Jia-xuan1,2,3,4*, HU Yu-ning1, YUE Xiang-lei4,5, WANG Lei4,5, LIU Zhe5

(1.School of Architecture and Civil Engineering, Xi’an University of Science and Technology, Xi’an 710054, China；2.Post-doctoral Research Center of Mining Engineering, Xi’an University of Science and Technology, Xi’an 710054, China；3.Ecology and Environment Bureau of Yulin, Yulin 719000, China；4.Research Institute of Membrane Separation Technology of Shaanxi Province, Xi’an University of Architecture and Technology, Xi’an 710055, China；5.School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China)., 2022,42(2)：665～671

Polyamide composite nanofiltration (NF) membrane was used to remove persistent organic pollutant-perfluorooctanoate acid (PFOA) from water. This research investigated the influence of PFOA concentration, ionic strength, pH value, typical macromolecular organic matter-bovine serum albumin (BSA), and the coexistence of BSA and Na+in feed solution on PFOA rejection, respectively. The filtration experiments were conducted at an operating pressure of 0.8MPa for 24h. Results showed that the retention of PFOA increased with increasing PFOA concentration. The higher the ionic strength or pH of the feed solution, the higher the rejection rate of PFOA was. When BSA presented in PFOA feed solution, BSA could not only block some membrane pores, but also form an organic fouling layer on the membrane surface which increased the negative charge of the membrane surface. Thus, both size exclusion and electrostatic repulsion abilities of the NF membrane were enhanced. When BSA coexisted with Na+in the feed water, the pore blocking degree of the NF membrane was more serious. The presence of Na+made the organic fouling layer much thicker. Therefore, the sieving capacity of the NF membrane was enhanced.

perfluorooctanoate acid；polyamide composite nanofiltration membrane；solution characteristics；bovine serum albumin；inorganic ions

X703.5

A

1000-6923(2022)02-0665-07

王佳璇(1990-),女,陜西渭南人,講師,博士,主要從事納濾膜法水處理理論與應(yīng)用研究.發(fā)表論文20余篇.

2021-07-21

西安科技大學(xué)博士啟動(dòng)基金資助項(xiàng)目(2018QDJ023);國家自然科學(xué)基金青年基金資助項(xiàng)目(51808442,51808432);陜西省自然科學(xué)基礎(chǔ)研究計(jì)劃-一般項(xiàng)目(青年)(2019JQ-529);榆林市科技計(jì)劃項(xiàng)目(2019-136,CXY-2020-101);中國博士后科學(xué)基金資助面上項(xiàng)目(2020M683679XB)

* 責(zé)任作者, 講師, wangjiaxuan90@126.com